A hammer drill includes a tool holder in which a cutting tool, such as a drill bit, can be supported and driven by the hammer drill. The hammer drill can often drive the cutting tool in three different ways, each being referred to as a mode of operation. The cutting tool can be driven in a hammer only mode, a rotary only mode and a combined hammer and rotary mode. A hammer drill will typically comprise a housing, an electric motor mounted in the main housing and a transmission mechanism by which the rotary output of the electric motor can either rotationally drive the cutting tool to perform the rotary only mode or repetitively strike the end of a cutting tool to impart axial impacts onto the cutting tool to perform the hammer only mode or rotationally drive and repetitively strike the cutting tool to perform the combined hammer and rotary mode.
In such types of hammer drill, a hollow spindle is typically mounted within the housing which can be rotationally driven by the electric motor. The spindle can be made in a one piece construction or from a number of component parts connected together. A tool holder, which holds the cutting tool, is firmly attached to the front end of the spindle. The electric motor rotatingly drives the cutting tool held within the tool holder by rotatingly driving the spindle within the housing via a gearing in the transmission.
A piston is typically mounted within the spindle and is connected to the motor via a reciprocating drive mechanism such as a crank mechanism or wobble bearing or similar such device within the transmission mechanism. A beat piece is mounted within the spindle towards the front of the spindle. The beat piece is capable of being axially slid over a limited range of movement. Also mounted within the spindle, in an axially slideable manner, between the piston and the beat piece is a ram.
The motor causes a cutting tool held in the tool holder to be struck by rotatingly driving the reciprocating drive mechanism to cause the piston to be reciprocatingly driven within the spindle. The piston in turn reciprocatingly drives the ram via an air cushion, the ram repetitively striking the beat piece as it does so. The beat piece transfers the impacts from the ram to the cutting tool. The design and operation of such hammer mechanisms is well known in the art and will not be described in any more detail. European Patent Publication No. EP1157788 describes an example of such a hammer drill.
The tool holder can be permanently attached to the front end of the spindle of the hammer drill. In such designs, the end of the spindle may form a component part of the tool holder with the spindle supporting a part of the cutting tool which is inserted into the end of the spindle. However, this restricts the range of cutting tools which can be used with the hammer drill as the tool holder will only be able to support certain cutting tools having specific connection arrangements which are compatible with that particular tool holder.
In order to broaden the range of tool cutting tools which can be used with a particular hammer drill, it is desirable to make the tool holder releasably connectable to the spindle. By making the tool holder releasably connectable, a number of different tool holders can be supplied with each individual hammer drill to enable that hammer drill to be used with a greater range of cutting tools. In this manner, one tool holder having one type of connection system can be removed from the spindle and replaced with another tool holder having a different connection system. European Patent Application No. EP1619000 discloses an example of a tool holder which can be releasably attached to a spindle.
One of the problems associated with releasably attachable tool holders is ensuring that the connection to and removal from the spindle is kept simple while maintaining a secure connection of the tool holder with the spindle when it is mounted on the spindle. Another problem with known attachable tool holders is that they significantly increase the overall length of the hammer drill when attached to the spindle.